1. Field of the Invention
The present invention relates to radio frequency (RF) communication systems and, more particularly, to a circuit for controlling a second order intercept point (IP2) in a mixer of a direct conversion receiver.
2. Description of the Related Art
In a receiver employing a superheterodyne architecture, a third order intermodulation (IM3) is significant. When a carrier signal is modulated into a baseband signal of a desired frequency band to be transmitted or received, non-linearity of a device (e.g., mixer) with multiple input frequencies causes undesired output frequencies that are different from the input frequencies. The input signals having two or more frequencies are mixed together produce distortion, i.e. an intermodulation distortion (hereinafter, referred to as IMD), having additional undesired frequencies. When input signals having two input frequencies pass through a non-linear device, intermodulation (hereinafter, referred to as IM) components are generated. The IMD is caused by the IM components. The IM components have frequencies corresponding to the sum of the two input frequencies and the difference between the two input frequencies. Thus, when two input signals having two different input frequencies are applied to the non-linear device, the IMD causes interference with modulation and demodulation.
When the frequency of the carrier signal is converted to an intermediate frequency (IF) in a superheterodyne conversion process, a third order IMD can occur at baseband frequencies and thus cannot be easily filtered out. Direct conversion (also called zero-IF or homodyne) is a special case of the superheterodyne receiver. In this case, the local oscillator LO is set to the same frequency as the desired RF channel. That means that the IF is zero, or dc. Now the filtering and gain can take place at dc, where gain is easier to achieve with low power. The basic operation of a direct-conversion receiver can be described as mixing an input signal frequency of (fRC+Δ), where (Δ) is the bandwidth of the modulation, with a local oscillator at fLO, yielding an output at: fMIXOUT=(fRF+Δ−fLO) and (fRF+Δ+fLO). In a conventional superheterodyne receiver, second-order distortion terms usually fall out of band and can be easily filtered. However, in a direct-conversion receiver, even-order distortion, particularly second-order products, will cause in-band interference.
In a direct conversion receiver, the received carrier signal is directly down-converted to the baseband signal, and so a second order IMD occurs at baseband frequencies. Thus, in the direct conversion receiver, the second order IMD has more effect on a signal distortion than the third order IMD, and accordingly there is a need for adjusting the second order IMD to prevent the signal distortion.
The theoretical point where the linear extension of the second order IMD intersects the linear extension of an input signal is referred to as a second order intercept point (IP2). The IP2 is an important parameter used to characterize a radio frequency (RF) communication system, and represents the total non-linearity of the communication system. As the value of the intercept point increases, the device has less non-linearity.
As the power level of the input signal is increased, the power level of the second order IMD at the output is also increased, and the point where the power level of the second order IMD intercepts the original power level of the input signal represents the IP2. However, since the output power is generally saturated before the output power reaches a theoretical IP2 point, a real IP2 point corresponds to only an expected hypothetical output power level where the second order IMD is expected to reach the same amplitude level as the input power level.
The linearity of the communication system may be increased by achieving a high IP2, which reduces the second order IMD (IM2). In general, a mixer in a direct conversion receiver has an IP2 calibration circuit for adjusting the IP2.
FIG. 1 is a circuit diagram illustrating a conventional second order intercept point (IP2) calibration circuit.
Referring to FIG. 1, the IP2 calibration circuit includes a mixer 100 and an IP2 modulator 102. The conventional IP2 calibration circuit of FIG. 1 is described by K. Kivekas et al., in “Calibration techniques of active BiCMOS mixers”, IEEE J. Solid-State Circuits, June 2002, Vol. 37, pp. 766-769, which is incorporated herein by reference in its entirety.
The mixer 100 includes a first pair of input terminals 104 for receiving a carrier signal VRF and a second pair of input terminals 106 for receiving a local oscillation signal VLO. The mixer 100 outputs a frequency difference (e.g., fRF+Δ−fLO) between the frequency of the carrier signal VRF and the frequency of the local oscillation signal VLO. The output signal of the mixer 100 is output to a pair of output terminals 108.
The IP2 controller 102 includes load resistors RLP, RLN and a calibrating resistor Rcal. The calibrating resistor Rcal is connected in parallel to the load resistors RLPand RLN. The calibrating resistor Rcal compensates for a mismatch between differential outputs Vop and Von of the mixer 100. A total second order intermodulation (IM2) output voltage is obtained by summing up the IM2 output voltage in a common mode and the IM2 output voltage in a differential mode.
The IM2 output voltage VIM2,cm in the common mode is given by the following expression 1:VIM2,cm=icm(R+ΔR−Rc)−icm(R−ΔR)=icm(2ΔR−Rc),   <Expression 1>wherein RLN is represented by (R−ΔR), and Rc denotes a decrease in the resistance value of RLP (e.g. RLP=R+ΔR) due to Rcal, and icm represents a current in a common mode.
The IM2 output voltage VIM2,dm in the differential mode is given by the following expression 2:VIM2,dm=idm(R+ΔR−Rc)+idm(R−ΔR)=idm(2R−Rc),   <Expression 2>wherein RLN is represented by (R−ΔR), and RC represents a reduction of the resistance value of RLP (e.g. RLP=R+ΔR) due to Rcal, and idm represents a current in a differential mode.
Therefore, the total IM2 output voltage VIM2 is given by the following expression 3:VIM2=VIM2,cm+VIM2,dm=idm(2R−Rc)+icm(2ΔR−Rc).   <Expression3>
The second order intercept point (IP2) is calibrated by adjusting RC (e.g., by changing Rcal), to change (e.g., to reduce) VIM2. The use of the above-mentioned calibration-method (using the resistor Rcal) has limitations in a semiconductor manufacturing process. Since ΔR is in a range of from about 0.1% to 10% of R, RC is also in a range of from about 0.1% to 10% of R. Therefore, the Rcal needs to be ten times to thousand times as large as the resistance of R. Thus, when R is tens of KΩ, Rcal needs to be tens of MΩ. Therefore, it is difficult to implement Rcal in a semiconductor manufacturing process, since a considerably large resistor occupies a large area on a semiconductor substrate and additional logic circuits are required. In addition, when the resistive load is used in IP2 calibration, a sufficient voltage margin may not be acquired in a structure where a high gain and linearity is required.